1. Field of the Invention
The present invention relates to a liquid crystal display device constructed by filling liquid crystals into the space between a pair of substrates. In particular, the present invention relates to a multi-domain vertical alignment (MVA) liquid crystal display device which is provided with domain regulation structures for forming, in each picture element, a plurality of areas (domains) where the directions of liquid crystal molecules are different from each other.
2. Description of the Prior Art
Liquid crystal display devices have the advantages that they are thin and light and that they can be driven at low voltages to have low power consumption. Accordingly, liquid crystal display devices are widely used in various kinds of electronic devices. In particular, active matrix liquid crystal display devices in which a thin film transistor (TFT) is provided for each picture element are comparable in display quality to cathode-ray tube (CRT) displays, and are therefore widely used in televisions and displays for personal computers and the like.
However, a general liquid crystal display device is inferior in viewing angle characteristics in comparison with CRT displays. In other words, contrast and color greatly change depending on whether a screen is viewed from the front thereof or from an oblique direction.
As a liquid crystal display device which is excellent in viewing angle characteristics, MVA liquid crystal display devices have been conventional (Japanese Patent Publication No. 2947350 (Patent Literature 1) and Laid-open Japanese Patent Publication No. 2002-107730 (Patent Literature 2)).
FIG. 1 is a plan view showing a picture element part of an MVA liquid crystal display device. FIG. 2 is a schematic cross-sectional view of the same MVA liquid crystal display device. Incidentally, liquid crystal display devices include a transmission type liquid crystal display device in which a backlight is used as a light source to perform display by use of light passing through a liquid crystal panel, a reflection type liquid crystal display device in which display is performed by use of the reflection of outside light (natural light or lamplight), and a semi-transmission type liquid crystal display device in which a backlight is used where it is dark and in which display is performed by use of the reflection of outside light where it is bright. Here, a transmission type liquid crystal display device will be described.
An MVA liquid crystal display device has first and second substrates 10 and 20 made of transparent thin plates of glass or the like, and a liquid crystal layer 30 made of nematic liquid crystal with negative dielectric anisotropy which fills the space between these substrates 10 and 20. On the substrate 10, as shown in FIG. 1, a plurality of gate bus lines 11a and a plurality of auxiliary capacitance bus lines 11b horizontally extending and a plurality of data bus lines 13 vertically extending are formed. The gate bus lines 11a and the auxiliary capacitance bus lines 11b are placed alternately with respect to the vertical direction.
Each of the rectangular areas surrounded by the gate and data bus lines 11a and 13 is defined as a picture element (sub-pixel) area. As described later, one pixel P is constituted by three picture elements of a red (R) picture element, a green (G) picture element, and a blue (B) picture element, which are placed along a horizontal direction.
On the substrate 10, a thin film transistor (TFT) 14, an auxiliary capacitance electrode 15, and a picture element electrode 16 are formed for each picture element area. In this liquid crystal display device shown in FIG. 1, a part of the gate bus line 11a is used as the gate electrode of the TFT 14. Moreover, the drain electrode 14d of the TFT 14 is connected to the data bus line 13. The picture element electrode 16 is made of transparent conductive material, such as indium-tin oxide (ITO) or the like, and electrically connected to the source electrode 14s of the TFT 14 and the auxiliary capacitance electrode 15 through contact holes C1 and C2. Each auxiliary capacitance electrode 15 is formed at a position where it is opposed to the auxiliary capacitance bus line 11b. 
Hereinafter, a layered structure on the first substrate 10 will be described with reference to FIGS. 1 and 2.
The gate bus lines 11a and the auxiliary capacitance bus lines 11b are formed in the same layer. On the gate bus lines 11a and the auxiliary capacitance bus lines 11b, a first insulating film (gate insulating film) 12a is formed. On predetermined areas of the first insulating film 12a, semiconductor layers (not shown) to be active layers of the TFTs 14 are formed. On these semiconductor layers, channel protection films (not shown), and the source and drain electrodes 14s and 14d of the TFTs 14 are formed. Moreover, on the first insulating film 12a, the data bus lines 13 and the auxiliary capacitance electrodes 15 are formed.
The data bus lines 13, the auxiliary capacitance electrodes 15, the source and drain electrodes 14s and 14d are covered with a second insulating film 12b. On this second insulating film 12b, the picture element electrodes 16 are formed.
On the picture element electrodes 16, a plurality of protrusions (banks) 17 extending in oblique directions relative to the data bus lines 13 are formed. These protrusions 17 are formed of, for example, photoresist, and are bent at portions where the protrusions 17 intersect the gate bus lines 11a and the auxiliary capacitance bus lines 11b. Moreover, the surfaces of the picture element electrodes 16 and the protrusions 17 are covered with an alignment film 18 made of polyimide or the like.
Hereinafter, a layered structure on the second substrate 20 will be described with reference to FIG. 2.
On the surface of the substrate 20 which faces the liquid crystal layer 30, a black matrix (shading film, not shown) made of, for example, Cr (chromium) and color filters 22 are formed. The black matrix is formed in portions where it is opposed to the gate and data bus lines 11a and 13 and the TFTs 14. Further, in each picture element area, a color filter 22 of any color of red (R), green (G), and blue (B) is placed to be opposed to the picture element electrode 16. In this example, one pixel P is constituted of three picture elements aligned horizontally, which are a red picture element in which a red filter is placed, a green picture element in which a green filter is placed, and a blue picture element in which a blue filter is placed.
On the color filters 22 (under the color filters 22 in FIG. 2), an opposing electrode (common electrode) 23 made of transparent conductive material, such as ITO or the like, is formed. On the opposing electrode 23, protrusions 24 are formed. The protrusions 24 are formed of, for example, photoresist, and placed between the protrusions 17 on the substrate 10 as shown in FIG. 2. The surfaces of the opposing electrode 23 and the protrusions 24 are covered with an alignment film 25 made of polyimide or the like.
The substrates 10 and 20 are placed in a manner such that the surfaces thereof on which the alignment films 18 and 25 are formed are opposed to each other. Into the space between these substrates 10 and 20, liquid crystals (liquid crystal layer 30) are filled. The structure constructed by filling the space between the substrates 10 and 20 with the liquid crystal is hereinafter referred to as a liquid crystal panel. Moreover, the substrate (substrate 10 in this example) on which the TFTs 14 are formed is referred to as a TFT substrate, and the substrate (substrate 20 in this example) placed to be opposed to the TFT substrate is referred to as an opposing substrate.
Two polarizing plates (not shown) are placed with the liquid crystal panel interposed therebetween in the state where the absorption axes thereof are orthogonal. Moreover, the liquid crystal panel is connected to a driving circuit, and display signals (R, G, and B signals) and scan signals are supplied from the driving circuit.
In the liquid crystal display device configured as described above, liquid crystal molecules 30a are vertically aligned with the surfaces of the alignment films 18 and 24 in the state where a voltage is not applied to the electrodes 16 and 23. Accordingly, as shown in FIG. 2, though the liquid crystal molecules 30a in the vicinities of the protrusions 17 and 24 are obliquely aligned with the surfaces of the substrates, most liquid crystal molecules 30a are vertically aligned with the surfaces of the substrates. In this case, light entering the liquid crystal layer 30 from the bottom of the substrate 10 through one polarizing plate passes through the liquid crystal layer 30 without change in the polarization direction thereof, and is blocked by the polarizing plate on the substrate 20. That is, this case results in dark display.
On the other hand, when a voltage higher than a predetermined voltage (threshold voltage) is applied as a display signal to a data bus line 13 and a scan signal is supplied to a gate bus line 11a, the TFT 14 is turned on, and thus the display signal is written to the picture element electrode 16. This causes the liquid crystal molecules 30a between the picture element electrode 16 and the opposing electrode 23 to be obliquely aligned with the electric field as shown in FIG. 3. In this state, light entering the liquid crystal layer 30 from the bottom of the substrate 10 through the polarizing plate changes the polarization direction thereof in the liquid crystal layer 30 to pass through the polarizing plate on the substrate 20. That is, this case results in bright display.
It is possible to also display intermediate tones by adjusting a voltage applied to the picture element electrode 16. Further, It is possible to display a desired image on the liquid crystal display device by controlling a voltage applied to the picture element electrode for each picture element.
Since the protrusions 17 and 24 are provided in the above-described MVA liquid crystal display device, the tilt directions of the liquid crystal molecules 30a are different on both sides of each of the protrusions 17 and 24 as boundaries. In the case where the protrusions 17 and 24 are formed on the basis of a pattern as shown in FIG. 1, the tilt directions of the liquid crystal molecules 30a are different from each other among areas A1, A2, A3, and A4 as shown in FIG. 4. When multi-domain is thus achieved, the leakage of light in an oblique direction relative to the surfaces of the substrates is suppressed. Accordingly, viewing angle characteristics are significantly improved.
Further, in the above-described example, the description has been performed for the case where the protrusions 17 and 24 are used as domain regulation structures. However, slits provided in at least any one of the picture element electrodes 16 and the opposing electrode 23, or dents (grooves) provided in the substrate surfaces (electrodes or insulating film thereon) may be used as domain regulation structures.
Moreover, in Laid-open Japanese Patent Publication No. Hei 10(1998)-62623 (Patent Literature 3), a proposition is performed for improvement of viewing angle characteristics by forming an optical compensation layer inside a liquid crystal panel.
However, the inventors of the present application suppose that the aforementioned conventional MVA liquid crystal display device has the following problem. Specifically, in the aforementioned conventional MVA liquid crystal display device, there occurs the phenomenon (hereinafter referred to as discolor) in which a portion with a low brightness looks whitish when a screen is viewed from an oblique direction.